The outer surfaces of aircraft components such as fuselages, wings, tail fins, engine nacelles, and the like, are typically constructed from non-metal composite materials, aluminum, or hybrid materials that include a combination of composite materials and metal. When lightning strikes a metal outer skin of an aircraft, the metal skin provides a highly conductive path that permits an electrical current to pass across the metal skin from a lightning strike point to a lightning exit point without substantial damage to the surface of the aircraft. Many modern aircraft components such as engine nacelles, however, are constructed of strong but light-weight composite materials that help to minimize the overall weight of the aircraft. These composite materials often comprise carbon or graphite reinforcement fibers distributed within a polymeric matrix. Such composite structures typically are substantially less electrically conductive than metal structures, and are substantially less capable of distributing and dissipating electrical energy resulting from a lightning strike. Accordingly, external surfaces of such composite aircraft components often include lightning strike protection that provides a highly conductive electrical path along their external surfaces. Such a conductive path permits the electrical energy associated with a lightning strike to be rapidly dissipated across the protected surface, which helps minimize damage to the surface of the aircraft component at the lightning strike point.
Airworthiness certification authorities have established standards for lightning strike protection for various types of aircraft and aircraft components. Based upon the probability of a lightning strike to a particular portion of an aircraft and the probable intensity of the electrical energy generated by such a strike, authorities have designated various potential strike zones for each type of aircraft and the probable current waveforms that structures and systems within each zone must withstand without substantial damage. Authorities designate these different strike zones as Zones 1A and 1B, Zones 2A and 2B, and Zone 3. These various strike zone designations are described in U.S. Pat. No. 5,417,385 and SAE ARP 5414, for example, and are understood by persons skilled in the art.
Composite aircraft components which are classified as Zone 1A require the greatest degree of lightning strike protection. SAE ARP 5416 sets forth lightning strike test procedures for certifying Zone 1A aircraft components. In order to satisfy the requirements of SAE ARP 5416, a test panel that replicates the structure of the Zone 1A component must withstand an artificially produced lightning strike having a specified current wave form without penetration through the test panel.
Current lightning strike protection systems for non-metal composite aircraft structures typically comprise a lightning strike protection surface film that includes a metal foil or mesh that is disposed on or proximate to an external surface of the composite structure to facilitate the distribution and dissipation of electrical energy generated by a lightning strike on the protected surface. For example, a metal foil or mesh can be embedded within a thin layer of a polymeric material that is disposed on a surface of a composite structure. One process for bonding a metal foil or mesh on a surface of a laminated composite structure for lightning strike protection is described in U.S. Pat. No. 5,470,413, for example. Alternatively, a metal foil or mesh can be incorporated into a surface portion of a laminated composite structure as the structure is fabricated. For example, U.S. Pat. No. 5,417,385 describes fabricating a laminated composite structure with a metal foil or mesh disposed proximate to its outer surface. In order to provide a smooth and aerodynamic outer surface for painting, a thin polymeric surface layer can be provided over the surface film containing the metal foil or mesh.
One common type of lightning strike protection includes an aluminum foil or mesh that is disposed on or proximate to the external surface of a protected composite structure. In one example, an aluminum foil or mesh that is capable of satisfactorily protecting a Zone 1A component has an areal weight density of about 0.02 pounds per square foot or about 74 grams per square meter (gsm). The term “areal weight density” is commonly associated with thin materials such as fabric, tape, foils and the like, and is well known among persons skilled in the art. As used herein, “areal weight density” refers to the weight of the material divided by its area (for example, length times width of a rectangular piece) of the material. The total areal weight density of conventional lightning strike protection systems (aluminum foil or mesh, polymer matrix, and a fiberglass corrosion isolation layer) can be about 0.11 pounds per square foot or about 500 gsm, or less.
Though an aluminum foil or mesh like that described above has proven to be effective for lightning strike protection for Zone 1A components, such a metal foil or mesh can add undesired weight to an aircraft. In addition, differences in the coefficients of thermal expansion (CTEs) between the metal foil or mesh and the polymers and reinforcement materials to which it is attached can introduce thermal stresses in the individual constituents. As a result, the protected surface can become prone to microcracking when subjected to repeated variations in ambient temperature routinely experienced by aircraft during service. At high altitudes, an aircraft (including its external components) is often exposed to relatively low ambient temperatures, whereas on the ground, the aircraft is exposed to relatively high ambient temperatures. These cyclic variations in temperature can be substantial. When a metal foil or mesh and the surrounding polymeric material have different CTEs, such variations in temperature can induce differential thermal expansion between the metal and the associated composite structure, and the resulting thermal stresses can cause microcracks to form in the surface of the composite structure. Such microcracks are undesirable as they can permit the ingress of moisture or chemicals into the composite structure and cause the structure to degrade.
Aircraft manufacturers and their suppliers are continually looking for ways to reduce the weight of their product for a number of reasons, including greater range per unit of fuel and/or greater fuel efficiency. Accordingly, there is a need for an improved surface film for lightning strike protection for Zone 1A aircraft components that is lighter in areal weight density than known surface films that include metal foils or screens. In particular, there is a need for an improved surface film for lightning strike protection of Zone 1A components that has an areal weight density less than about 500 gsm. In addition, there is a need for improved durability of the surface film for Zone 1A lightning strike protection, and for one that is less likely to be prone to surface microcracking than surface films that include metal foils or screens.